Nonreciprocal absorption filter

ABSTRACT

A nonreciprocal absorption filter (i.e., narrow band isolator) is disclosed comprising a magnetically biased gyromagnetic element located off the center axis of a conductively bounded waveguide. For one direction of wave propagation, the gyromagnetic element forms a dielectric resonator which is critically coupled to the signal and absorbs wave energy over a narrow band of frequencies within a given, broader band of interest. For wave propagation in the opposite direction, the field displacement effect produced by the element decouples the signal from the cavity and substantially no absorption takes place.

United States Patent [1 1 Kostelnick Nov. 11, 1975 NONRECIPROCAL ABSORPTION FILTER [75] Inventor: Joseph John Kostelnick, Bethlehem, Pnmm'y Gensler Pa- Attorney, Agent, or F1rm-S. Sherman [73] Assignee: Bell Telephone Laboratories.

Incorporated, Murray Hill. NJ. [57] ABSTRACT A nonreciprocal absorption filter (i.e., narrow band 7 [*2] Flled' June 1974 isolator) is disclosed comprising a magnetically biased [21] Appl. No.: 476,696 gyromagnetic element located off the center axis of a conductively bounded waveguide. For one direction of wave propagation the gyromagnetic element forms a 3 ;5 z dielectric resonator which is critically coupled to the l I I o I I v a I o s I l e I l Q [58] Field of 333/24 73 81 B frequencies within a given. broader band of interest. For wave propagation in the opposite direction. the [56] References cued field displacement effect produced by the element de- UNITED STATES PATENTS couples the signal from the cavity and substantially no 2.963.667 12/1960 Hewitt. Jr. 333/242 absorption takes place. 3327.251 6/1967 Degan. Jr i i. 333/242 3,548,343 12/1970 Kostelnick 333/242 6 Chums 9 Drawing Flgures US. Patent Nov. 11, 1975 Sheet 1 of3 3,919,673

.fz FREQUENCY l 2 FREQUENCY NONRECIPROCAL ABSORPTION FILTER This invention relates to nonreciprocal filters.

BACKGROUND OF THE INVENTION Nonreciprocal attenuators. i.e.. isolators, using gyromagnetic materials are well known in the art. See. for example. Behavior and Applications of Ferrites in the Microwave Region by A. G. Fox et al., published in the January 1955 issue of the Bell System Tee/mica] Journal, pages 5403. Among the several types of isolators described by Fox et al. are the resonance isolators and the field displacement isolators.

In a resonance isolator, (as described in U.S. Pat. No. 3,076,946), a vane of gyromagnetic material, located in the signal path, is magnetically biased to gyromagnetic resonance. Because of the nonreciprocal nature of the loss component of its permeability, the resonantly biased material absorbs relatively little wave energy from signals propagating in the socalled forward direction, while absorbing substantial amounts of energy from signals propagating in the so-called reverse direction.

It is a disadvantage of a resonant isolator that the magnetic biasing field required is proportional to the operating frequency, and becomes excessive at the higher microwave frequencies and above. 7

In the field displacement isolator. (as described in US. Pat. No. 2.834.945) the nonreciprocal nature of the real part of the permeability of gyromagnetic mate rials is utilized to modify the field configuration of the signal for opposite directions of wave propagation. By suitably locating a vane of lossy material in the wavepath, along with the gyromagnetic element. energy is absorbed in the reverse direction of signal propagation to a much greater extent than in the forward direction of signal propagation due to the differences in the field configurations.

It is an advantage of the field displacement isolator that it operates with a much smaller magnetic biasing field than the resonance isolator. However, the lossy member, while nominally located in a region of an electric field null for wave energy propagating in the forward direction, nevertheless tends to absorb a small, but finite amount of signal energy. In addition, field displacement isolators tend to be broad band.

The broad object of the present invention is to obtain high reverse-to-forward loss ratio isolators using small gyromagnetic elements and low biasing fields.

It is a further object of the present invention to reduce the absorption bandwidth of gyromagnetic isolators.

SUMMARY OF THE INVENTION A narrow band isolator, in accordance with the present invention, comprises a partial height cylinder of'gyromagnetic material mounted on one of the wide (H- plane) walls of a rectangular. conductively-bounded waveguide. The axis of the cylinder is oriented normal to said walls, and is located to one side of the longitudinal axis of the guide. A magnetic field, directed parallel to the cylinder axis, biases the gyromagnetic material well below gyromagnetic resonance for the operating frequencies of interest.

In such an arrangement, the gyromagnetic material serves two purposes. In the first instance it servesto.

produce a field displacement effect, as in the prior art field displacement isolators, such that the field configurations are different for oppositely-propagating waves. Secondly. the gyromagnetic element serves as a'dielectric resonator that is critically coupled to the signal for one of the two directions of propagation. and operates as a narrow-band absorptive filter. In the opposite direction of wave propagation. a different field configuration exists such that the signal is decoupled from the cavity and no significant absorption takes place.

It is a first advantage of the invention that a resonance absorption is produced in a gyromagnetic element with a magnetic biasing field that is substantially less than that required to produce gyromagnetic resonance.

It is a second advantage of the invention that while the field displacement effect is utilized. there is no inherently lossy material included within the wavepath as in prior art field displacement isolators.

As a further consequence of the fact that no inherently lossy material is included within the \vavcpath. the total field displacement effect required in an isolator in accordance with the present invention is much less than in the prior art field effect isolator. As a result much smaller magnetic biasing fields can be used.

It is another advantage of the invention that very narrow absorption bands can be realized.

It is a final advantage of the invention that a very much smaller element of gyromagnetic material is usecl as compared to prior art isolators.

As a result ofv all of the above-enumerated advantages. inexpensive, narrow-band isolators having high reverse-loss to forward-loss ratios are realized.

These and other objects and advantages. the nature of the present invention. and its various features. will appear more fully upon consideration of the various illustrative embodiments now to be described in detail in connection with the accompanying drawings.

BRIEF DESCRIPTION 0F THE DRAWINGS FIG. 1 shows an absorption filter in accordance with the present invention:

FIG. 2 shows the transmission loss characteristic produced by a dielectric resonator located along the center of a conductively bounded waveguide;

FIG. 3 shows the transmission loss produced by a dielectric cavity asymmetrically located in a conductively bounded waveguide;

FIG. 4 shows a first specific embodiment of the invention;

FIGS. 5 and 6 show the transmission loss characteristic of the embodiment of FIG. 4 on two frequency scales;

FIG. 7 shows a second specific embodiment of the invention:

FIG. 8 shows the transmission loss characteristic of the embodiment of FIG. 7; and

FIG. 9 shows a cascade of two filters in accordance with the present invention.

DETAILED DESCRIPTION Referring to the drawings. FIG. 1 illustrates a nonreciprocal absorption filter. in accordance with the present invention, which employs the field displacement effeet to produce nonreciprocal operation The filter in- .cludes a section 10 of conductively bounded electrical transmission line for guiding electromagnetic wave energy which,.for purposes of illustration, is a length of rectangular waveguide of the metallic shield type having a wide internal cross-sectional dimension of at least one-half wavelength for the lowest frequency wave energy to propagate therein. and a narrow dimension of about one-half the wide dimensiornSo proportioned. the wave energy propagates in the dominant mode. known in the art as the TE mode. in which the electric field extends transversely across the guide in a direction normal to the wide guide walls. The intensity of the electric field in an unloaded guide varies sinusoidally along the wide dimension. having a maximum at the center of the guide andbeing substantially zero at the narrow walls.

Disposed within guide 10. and transversely displaced from the center thereof. is an element 11 of material capable of exhibiting gyromagnetie properties over the frequency band of interest. The term gyromagnetic material" is employed here in its accepted sense as designating the class of magnetically polarizable materials having unpaired spin systems involving portions of the atoms thereof that are capable of being aligned by an external magnetic polarizing field. and which exhibit a significant precessional motion at the frequencies within the range contemplated by the invention under the combined influence of said polarizing field and an orthogonally directed. varying magnetic field component. Typical of such materials are the ferromagnetic materials including the spinels such as magnesium aluminum ferrite. aluminum zinc ferrite. and the garnetlike materials such as yttrium iron garnet.

Element I l, as shown. is a circular cylinder mounted on one of the wide walls 12 of the waveguide with the cylinder axis normal thereto. The height h of the cylinder is less than the narrow dimension of the guide and. hence. there is a gap between the upper circular surface of the cylinder and the other wide guide wall 13.

Element 11 isbiased by a uniform constant magnetic field H directed in a dir ection parallel to the cylinder axis. As illustrated. in FIG. 1. this field is supplied by a permanent magnet 14 whose pole pieces N and S bear against the upper and lower wide walls. respectively. of guide 10. Alternatively. an electromagnct having a similar orother physical design can be used or the gyromagnetic material itself can be permanently magnetized.

In accordance with the present invention. gyromagnetic element 11 serves two simultaneous functions. First. it serves to produce a field displacement effect. Being located off the guide center. element 11 is located in a region or elliptically polarized magnetic field. Because of the nonreciprocal nature of the real part of the permeability constant of element 11, the resulting field configurations in the region of element 11 are different for opposite directions of wave propagation.

Secondly. element 11 serves as a dielectric resonator. It has been shown that suitably shaped dielectric members can be used as resonators in essentially the same way as conductively-bounded cavities. Theoretical and experimental results for microwave resonators formed from differently shaped dielectric materials have been described extensively by many workers in the field. See. for example. The Dielectric Microwave Resonator." by A. Okaya and L. F. Barash. published in the October 1962 issue of the Proceedings of the I.R.E., pages 2081-2092. For a description of the use of dielectric resonators in microwave bandpass filters. see Microwave Bandpass Filters Containing High-Q Dielectric Resonators, by S. B. Cohn. published in the 4 April 1968 issue of the I .LILIE. Trmtractions on Microwave Theory and Techniques. pages 21 8227. and other references cited therein.

Before considering the operation of the nonreciprocal filter illustrated in FIG. 1. a brief description of the interaction of a simple. dielectric cavity and a propagating wave is considered. If. for example. a dielectric cylinder is centrally located in waveguide 10 and the transmission loss measured as a function of frequency. a loss vs frequency curve 20. of the type illustrated in FIG. 2 is obtained. The large. broad transmission loss is due to a resonance condition established in the dielectric cavity. In the instant case, where the dielectric material was an unbiased ferrite member. this resonance produced a large reflected wave. In addition. in the instant case. a relatively small decrease in transmission occurred in a frequency range below the large resonance loss. as shown by the dotted curve portion 21. Of particular interest is the region between frequencies f and f wherein the transmission loss is a minimum.

As the dielectric cavity is displaced off-center. a second. much narrower absorption resonance is produced within the region between f and f This absorption peak is the result of a second mode of excitation produced by the displacement of the cavity. This is illustrated by the modified loss characteristic curve 22 in FIG. 3.

The exact location of the second absorption peak depends upon the physical dimensions of the dielectric material. its composition. and the particular mode in which the cavity is excited. The manner in which these parameters are related has been determined for a variety of materials. See. for example. A Mode Chart for Accurate Design of Cylindrical Dielectric Resonators by D. L. Rebsch. et al.. published in the-1.15.5.5 Transactions on Microwave Theory and Techniques. July 1965. In any particular case. reference can be made to such published data or similar data can be obtained experimentally.

It is apparent from FIG. 3 that a narrow-band absorption filter can be made using the second absorption resonance produced in the dielectric cavity. However. such a filter would be reciprocal, absorbing energy for either direction of propagation. To produce nonreciprocal transmission characteristics. the dielectric cavity, in accordance with the present invention, is made of a gyromagnetic material magnetically biased so as to couple between the propagating wave and the narrow band absorptive resonance for only one direction of wave propagation. In the reverse direction of wave propagation. the field displacement effect produced serves to prevent such coupling.

Referring once again to FIG. 1, the transverse position of element 11 is chosen so that the second resonance is critically coupled to the signal field for one direction of signal propagation. and essentially decoupled from the signal field for the opposite direction of signal propagation. For this one direction of propagation. the'desired resonance condition is thus established and the signal energy is absorbed by the gyromagnetic element. producing a relatively narrow absorption band. For the opposite direction of propagation, very little energy is coupled between the signal field and the second resonance and essentially all the signal energy is transmitted.

EXAMPLES Two embodiments of an isolator in accordance with the present invention were constructed. A first embodiment, designed to operate at GHZ, is illustrated in cross-sectional view in FIG. 4. Waveguide 10, as shown, had a wide dimension of 0.900 inches and a narrow dimension of 0.45 inches. Gyromagnetic element 11 had a diameter of 0.270 inches and a height of 0.350 inches.

The optimum magnetic biasing field and the optimum location of element 11 were determined empirically. The element was placed in different transverse positions within the guide, and the initial loss measurements made with no applied field. Under this condition it was noted that the loss increased, reaching a maximum at a particular displacement from the guide center. Without a field, however, the loss is reciprocal i.e.. the same for both directions of propagation. Having established a range of positions. element 11 was then repositioned closer to the 'guide center. and a biasing field applied. The loss measurements were then remade as both the field strength and the position of the element were varied until a maximum reverse-loss to forwardloss ratio is obtained. For the particular element used. (saturation magnetization 1291 gauss; dielectric constant 11.1; loss tangent 0.002) a displacement of 0.026 inches resulted in the transmission characteristic shown in FIG. 5 wherein the loss in the forward direction was about 0.5 db whereas the loss in the reverse direction was in excess of about -40 dB at about 9.930 MHZ.

Measurements made over a broader band between 9,140 and 10.590 MHz. shown in FIG. 6, indicated that the forward loss curve involved a relatively broad transmission band associated with a resonance effect which varied as a function of the diameter of the gyromagnetic element. Optimum performance is obtained when the absorption resonance and the transmission maximum peak at the same frequency. In FIG. 6, markers at 9,840 and 10.000 MHZ show the approximate frequency range included in FIG. 5.

A second isolator was constructed at 57.7 GHZ using WRIS millimeter waveguide. by sealing down all the dimensions of the lower frequency unit. Thus. the higher frequency unit, illustrated in FIG. 7, includes a 0.064 inch by 0.045 inch gyromagnetic element 40 disposed within a 0.148 inch 0.074 inch waveguide 41. Because the absorption resonance and the transmission maximum did not occur at the same frequency in this unit, a metallic tuning post was also included within the guide. The tuning post was found to have a greater effect upon the frequency of the absorption resonance than upon the transmission maximum. Hence, the post can be used to tune the former. and the diameter of the gyromagnetic element can be used to tune the latter. FIG. 8 shows the resulting loss characteristic for the particular gyromagnetic element (saturation magneti zation 5000 gauss; dielectric constant 12.5; loss tangent less than 0.001) dimensioned as shown in FIG. 7.

MULTIPLE RESONATORS It is often desirable in filter design to steepen the sides of the loss vs. frequency curve, and to flatten the transmission minimum. For this purpose two or more resonators. relatively closely spaced so as to couple electrically. are placed in the same transmission line. The nonreciprocal filter disclosed here can also be iterated in this fashion. with optimum coupling occurring when the ferrite cylinders are approximately one wavelength apart. as illustrated in FIG. 9.

In all cases it is understood that the above-described arrangements are illustrative of a small number of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can readily be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A nonreciprocal filter including:

a section of waveguide whose cross-sectional dimensions are proportioned to permit the propagation of electromagnetic wave energy over a defined frequency band of interest;

and means for attenuating wave energy within a selected portion of said band of interest for one direction of wave propagation comprising:

an element of gyromagnetic material magnetically biased below gyromagnetic resonance with respect to the frequency band of interest and positioned within said section to produce a region of substantial field intensity differential in said wave energy for opposite directions of wave propagation. and for simultaneously forming a dielectric resonator that is critically coupled to said wave energy over said selected portion of said band of interest for one direction of propagation while being substantially transparent to said wave energy for the opposite direction of wave propagation:

said section of waveguide being substantially free of any other attenuating means.

2. The filter according to claim 1 including a metallic post within said section of waveguide for tuning said element.

3. In combination:

a plurality of filters. each according to claim 1, longitudinally distributed along said waveguide such that each filter is spaced apart from the next adjacent filter a distance corresponding to one wavelength at the center of said. band of interest.

4. The filter according to claim 1 wherein said waveguide is a conductively bounded rectangular waveguide.

5. The filter according to claim 4 wherein said element of gyromagnetic material has a circular cylindrical shape;

and wherein said element is mounted on one of the wide walls of said waveguide with the cylinder axis normal thereto.

6. The filter according to claim 5 wherein said element is transversely displaced with respect to the center of said waveguide. 

1. A nonreciprocal filter including: a section of waveguide whose cross-sectional dimensions are proportioned to permit the propagation of electromagnetic wave energy over a defined frequency band of interest; and means for attenuating wave energy within a selected portion of said band of interest for one direction of wave propagation comprising: an element of gyromagnetic material magnetically biased below gyromagnetic resonance with respect to the frequency band of interest and positioned within said section to produce a region of substantial field intensity differential in said wave energy for opposite directions of wave propagation, and for simultaneously forming a dielectric resonator that is critically coupled to said wave energy over said selected portion of said band of interest for one direction of propagation while being substantially transparent to said wave energy for the opposite direction of wave propagation; said section of waveguide being substantially free of any other attenuating means.
 2. The filter according to claim 1 including a metallic post within said section of waveguide for tuning said element.
 3. IN combination: a plurality of filters, each according to claim 1, longitudinally distributed along said waveguide such that each filter is spaced apart from the next adjacent filter a distance corresponding to one wavelength at the center of said band of interest.
 4. The filter according to claim 1 wherein said waveguide is a conductively bounded rectangular waveguide.
 5. The filter according to claim 4 wherein said element of gyromagnetic material has a circular cylindrical shape; and wherein said element is mounted on one of the wide walls of said waveguide with the cylinder axis normal thereto.
 6. The filter according to claim 5 wherein said element is transversely displaced with respect to the center of said waveguide. 